1. Field of the Invention
The invention relates to a process for preparing protective-colloid-stabilized polymers in the form of their aqueous polymer dispersions or of their water-redispersible powders.
2. Background Art
Water-redispersible polymer powders, which are obtainable by drying the corresponding polymer dispersions, are known and have been used successfully for many years, particularly in the building sector. They improve the property spectrum of hydraulically setting systems, such as cement mortars, for example their abrasion resistance, their flexural strength in tension and their adhesion. Products of this type, which are available on the market, are usually prepared from polyvinyl acetate, vinyl acetate-ethylene copolymers, vinyl acetate-vinyl ester copolymers or vinyl chloride-ethylene copolymers.
Very high requirements have to be met if a dispersion powder is to be industrially useful: it must be free-flowing, it must not block when stored, i.e. its free-flowing nature must not be lost over time. If blocking of the powder occurs, it becomes practically impossible to handle. Once large agglomerates have formed, they cannot be mixed into the pulverulent formulation. To develop its full effectiveness, the powder must have very good redispersibility in water, giving the original particles of the dispersion.
Redispersion powders compete with dispersions in the individual application sectors and, for example in paints, the binding power of these binders depends in a known manner on particle size. For use in combination with hydraulic binders, such as cement, for improving mortar properties, a main application sector for redispersion powders, the formulations have to remain stable for a certain time and must not change their working consistency significantly (cement stability). This is because the user cannot be expected to remix at frequent intervals. A product of that type would be unacceptable. In the concrete and mortar industry a significant role is played by mechanical properties, such as compressive strength and porosity and the associated air-pore content. If too many air pores are present then there is a severe reduction in compressive strength, and if too few or no air pores are present in the mortar or concrete the building material has insufficient resistance to frost and condensation. In addition, the hydraulically setting systems modified with the dispersion powder should provide adhesion which is better still than that of unmodified systems.
Besides the dispersion powders mentioned, based on polyvinyl acetate, vinyl acetate-ethylene copolymers, vinyl acetate-vinyl ester copolymers or vinyl chloride-ethylene copolymers, there are also known powders based on styrene-butadiene polymers, on styrene-acrylate copolymers and on (meth)acrylate polymers, but the preparation processes and service properties of these are not fully satisfactory.
WO-A 96/17891 relates to the preparation of water-redispersible polymer powders based on vinyl acetate copolymers, styrene-butadiene copolymers and styrene-acrylate copolymers. These polymers are prepared by emulsion polymerization in the presence of conventional emulsifiers, and a mixture of saccharide, anionic alkylaryl emulsifier and polyvinylpyrrolidone is added before the polymer dispersion is dried. WO-A 96/20963 has disclosed a process for preparing water-redispersible polymer powders based on styrene-butadiene polymers, styrene-acrylate polymers and (meth)acrylate polymers. The polymers here are prepared in a two-step polymerization in the presence of emulsifier, giving core-shell polymers, and are dried by spray drying. WO-A 96/41825 likewise relates to dispersion powders based on core-shell polymers. The shell here has saccharide-functional comonomers and crosslinkable comonomers, for covalent linking of the shell to the core. Besides the relatively complicated procedure to prepare the redispersion powders, their service properties and specifically their workability (cement stability) are unsatisfactory due to their relatively small particle sizes.
The recommendation of EP-A 62106 (U.S. Pat. No. 4397968), in order to prepare aqueous dispersions of polyvinyl-alcohol-stabilized copolymers of (meth)acrylate or of styrene, is to improve the water-resistance of the polymers by metering in the main amount of monomers during the polymerization. The drying of the dispersions to give powders is mentioned. A disadvantage of the redispersion powders obtainable in this way is that because of their poor cement stability they cannot be used in cement-containing compositions: the workability period is too short, and even after a short time the compositions change their consistency and become progressively thicker and are no longer workable.
EP-A 538571 has disclosed the use of specific initiator systems for adjusting viscosity and hydrophilicity in preparing polymer dispersions stabilized with partially hydrolyzed polyvinyl alcohol. DE-A 1260145 recommends the use of modified polyvinyl alcohols for preparing finely divided polymer dispersions. A disadvantage is that this procedure only gives dispersions with severe foaming tendencies, a feature which is disadvantageous for their workability and service properties.
In WO-A 97/15603, no stable polymer dispersions are obtained from emulsion polymerization of hydrophobic monomers, such as styrene or butadiene, with stabilization by protective colloids. To obtain stable, protective-colloid-stabilized polymer dispersions from this monomeric starting material, polymerization in the presence of mercapto-functionalized, copolymerizable silanes is recommended. A disadvantage of this procedure is that it is essentially restricted to preparing silane-containing copolymers.
DE-A 4212768 describes the preparation of aqueous polymer dispersions based on styrene polymers, butadiene polymers and (meth)acrylate polymers and the drying of the dispersions to give dispersion powders. The polymerization takes place in the presence of a macromonomer made from a polyalkylene glycol esterified with maleic or fumaric acid.
DE-C 3590744 (GB-A 2181143) describes a process for preparing protective-colloid-stabilized polymers by polymerizing vinyl monomers in the presence of a protective colloid hydrophobicized with oxyalkylene units. A disadvantage in the use of polyvinyl alcohols hydrophobicized with oxyalkylene units is the plasticizing action of oxyalkylene units, which causes blocking and impaired redispersibility in powders produced therewith. The relatively polar character of the oxyalkylene units also markedly reduces water-resistance.
The object was therefore to provide a process for preparing protective-colloid-stabilized polymer dispersions and protective-colloid-stabilized water-redispersible polymer powders, essentially based on styrene-butadiene polymers, styrene-acrylate polymers and (meth)acrylate polymers, overcoming the disadvantages of the prior art described and permitting the preparation of stable polymer dispersions and of polymer powders which have good redispersibility and cement stability.